Lots of differing advice here, my opinion is that where possible the parents should marry.
Marriage is a covenant, based on a decision to commit to each other and love each other. If two people have already 'cut covenant' in the act of sex and have a child as a result and choose to marry and are determined to make it work then I believe God will honour this.
To be blunt, marriage decisions in the bible are not usually based on love and strong long term relationships, in most cases love is not even mentioned and often they barely knew each other. The love came AFTER the marriage, it came after the decision and the committment.
Todays society, romantic novels and Hollywood can be blamed for our culture thinking that the FEELING of love is what it is all about. Love is a decision and when you marry you decide to love that person and commit to love them for the rest of your life... whether you FEEL like it or not. YES absolutley there is a gift of attraction, all the 'ooshy gooshy' falling in love emotion, that high, the butterflies... but that isn't 'love' it's an emotional response to the attraction and doesn't usually last more than a few months.. it isn't really what love is about.
Love is choosing to still love and honour your spouse when you don't feel like it, when life is tough and you are getting on each others nerves, when you don't fancy them and your sex drive has dipped. Love is looking for ways to give 100% instead of 50/50, love is companionship, compassion, friendship but you have to choose to commit and then choose to remain committed no matter what your feelings say.
If two people are committed to making a marriage work, and make their vows in front of God and keep God in the marriage... the marriage will work, God will give them everything they need to have a happy successful marriage.
How do you know you have married the right person? .....It's the person you wake up with the day after your wedding. God loves marriage and upholds an honours it, whichever two people marry.